This invention relates to an electrostatic image developer used for developing latent electrostatic images in electrophotography, electrostatic recording, electrostatic printing, etc. More particularly, this invention relates to an electrostatic image developer with negative chargeability which can give clear image quality and high image density in spite of its small-diameter or the high content of fine toner particles.
There are well-known conventional developments of latent electrostatic image, and, for example in electrophotography, a uniformly charged photoconductor layer consisting of selenium, zinc oxide, a vinyl carbazole compound, cadmium sulfide, a phthalocyanine compound, etc. is subjected to light exposure with the same light image as depicted on a master drawing to extinguish the electrostatic charge on the exposed portions of the photoconductor to obtain a latent electrostatic image, on which a toner consisting of a binder resin, a coloring pigment and other additives is electrostatically deposited to form a toner image. The thus formed toner image is, as necessary, transferred to an image support such as paper, and then the toner thus transferred is fused by heating, softened or dissolved with a solvent or deformed by application of pressure to be permanently fixed onto the image support.
While various processes such as cascade development, powder cloud development, magnetic brush development, jumping development and touch down development are well known as the methods of latent electrostatic image development, the electrostatic image developers are roughly classified into two-component developer and single-component developer. The two-component developer consists of a toner and a carrier such as iron powders, steel beads, ferrites and glass beads, having a particle size larger than that of the toner, and the latent electrostatic image is developed by the toner charged through friction with the carrier. The single-component developer is further classified into magnetic single-component developer each of which consists of a toner and a magnetic substance such as triiron tetroxide, diiron trioxide and ferrite and forms a toner layer on a developer carrying member with the aid of the magnetic force and develops the latent electrostatic image; and nonmagnetic single-component developer which develops latent electrostatic images formed by the toner layers on developer carrying member by means of contact electrification, triboelectrification, etc.
As such toner, a small particle prepared by using a thermoplastic resin as the binder resin, and dispersing a coloring pigment, a charge control agent and other additives into the resin by melt kneading, followed by finely milling and classification of the resulting composite to properly regulate the particle size is generally employed. Further, there is known a developer prepared by adding to such toner other materials so as to impart properties necessary as the developer.
While vinyl resins such as polystyrenes, acrylate polymers, styrene-acrylate copolymers and styrene-butadiene copolymers; and polyesters, epoxy resins, polyamides, polyurethanes, polycarbonates, fluoropolymers, silicone resins, phenol resins, maleic resins and coumarone resins are so far known as the thermoplastic resin, particularly the polyesters among others are excellent in (a) chargeability, (b) fixing property, (c) transparency, (d) gloss and (e) plasticizer in vinyl chloride migration resistance, and are practically utilized as the binder resin encouragingly in recent years.
Meanwhile, British Patents Nos. 1,233,869 and 1,321,651 disclose electrostatic image developers, in which the toners are admixed with fluoropolymer fine particles, with a view to preventing adhesion of the toners onto the photoconductor, the so-called filming phenomenon, during repeated procedures of development.
What is most important in developing the latent electrostatic image using such electrostatic image developer is the quality of the image to be finally obtained, so that developers having excellent resolving power, high gradation and high image density are desired. Under such circumstances, studies and efforts have been made so as to cope with the requirements by using toners having smaller particle sizes. The conventional toners have an average particle size on the order of 10 .mu.m, and it is true that some effects can be identified for improving resolving power and gradation by using a toner having a smaller particle size, typically to an average particle size of 10 .mu.m or less, particularly 8 .mu.m or less.
But, when a latent electrostatic image is developed using such smaller-diameter toner, the image thus formed has a very low image density, disadvantageously, although it may have excellent resolving power and gradation. This is because the charge which the toner gains during the process of development is greatly changed by the reduced particle size of the toner. Namely, it is known that there is the following relationship: between the charge retained per toner particle and the particle size of the toner. In the above formula, q represents the charge (C) retained per toner particle; d represents the particle size (.mu.m) of the toner; and n is 1 to 2. Accordingly, the toner charge to mass (C/g) becomes considerably high as the particle size of the toner becomes smaller, in reverse proportion. Thus, when a latent electrostatic image is developed, the amount of the toner necessary for electrically neutralizing the latent image formed on the photoconductor is lowered. Namely, the great reduction in the image density is attributable to the reduction of the amount of the toner to be electrostatically deposited to the latent image to form an image with a small level of toner deposition.
In order to obtain a small-diameter toner which can overcome the above problem, the toner charge to mass must be reduced. However, for obtaining such small-diameter toner satisfying the above requirements, the toner materials to be contained in the toner such as the binder resin, coloring pigment, charge control agent and other additives or the percentage composition thereof must be modified; the respective materials must newly be designed; or the material design and formulation design must be reconsidered. However, if a polyester resin having excellent charge stability is employed as the binder resin, it is difficult to reduce the toner charge to mass due to the excellent charge stability. Thus, the toner image formed using the polyester as the binder resin suffers a problem that the images formed thereby have extremely low image density. While it has been attempted to reduce the toner charge to mass by optimizing the acid value of the polyester resin by changing the carboxylic acids or alcohols to be applied as the monomer to the resin or by changing the percentage composition of the monomer, it proved difficult to satisfy various properties required of the toner including thermal properties, mechanical properties, etc. and also to reduce the charge on a great margin.
Further, fine toner particles contained in the toner, typically those having a diameter of 4 .mu.m or less greatly reduce the image density due to the same reasons as described above. Namely, a large amount of fine particles contained in the toner increases the toner charge to mass and reduce the amount of the toner necessary for electrically neutralizing the latent image formed on the photoconductor, leading to the lowering of the image density. A classification step has conventionally been incorporated into the process of producing a toner so as to remove the fine particles. However, in the process of producing a small-diameter toner, it has been difficult to fully remove, in the classification step, a large amount of fine particles formed during the pulverizing step due to the limitation of the classification apparatus employed in the former step, and thus it has been extremely difficult to obtain a small-diameter toner which has a narrow particle size distribution and thus can give high image density. Meanwhile, if the cutpoint of classification is shifted to the larger particle side in the classification step so as to fully remove the fine particles, the toner yield is greatly reduced and the average particle size is increased, disadvantageously. Particularly when a toner containing as the binder resin a polyester is to be produced, the fine particle content tends to be higher compared with the case where a toner containing a styrene-acrylic resin as the binder resin is produced. Thus, it is difficult to obtain a toner image having high image density using such small-diameter toner containing a polyester resin.
Further, in the electrostatic image developer disclosed in the above-described patent literatures incorporated herein as reference, the fluoropolymer fine particle is allowed to present in the toner in an independent and free form, and intended solely to prevent filming. In addition, the lowering in the image density which occurs when a latent electrostatic image is developed using a small-diameter toner cannot be overcome by simply mixing the toner with the fluoropolymers as described in the above patent literatures.